Screw presses (screw compressors) are used for compression and/or dewatering of bulk matter. The purpose of this may be to attain a decrease in volume or to separate solid matter from liquid. A typical screw press comprises a rotatable screw that is encased in a casing. Screw presses are shown in SE 178 147 and U.S. Pat. No. 4,838,995, for example. In a screw press, it is desirable that the matter that passes through the screw press is brought to pass straight through the screw press without following the rotation of the screw. In order to prevent matter from following the rotation of the screw, it has been suggested in the above mentioned SE 178 147 to use counterstays inserted between the screw threads, in order to prevent a rotation tendency in the pulp that is fed through a screw press. Certain types of bulk matter may contain substances that are to be sorted out or separated from the rest of the matter. Certain types of waste matter may e.g. contain residues of fat and protein that is desirable to wash out, because organic matter may be worthwhile to recover. Domestic waste matter is advantageously broken up and diluted with water before compression, in order to achieve an extra washing effect. In paper pulp, it may be desirable to remove residual chemicals, acids or residual alkali for example. Yet another screw press for removal of chemicals from pulp is known from U.S. Pat. No. 2,355,091. This prior art device comprises a screw that is placed inside a casing that has an outer wall and an inner wall. It is stated that the inner wall consists of a plate that is pervious to liquid. A channel is formed between the screw and the inner wall, the cross-sectional area of which being progressively reduced in a direction towards an outlet end of the screw press. In one section of the screw however, the depth of the screw increases so that the cross-sectional area of the channel suddenly increases. In this section, liquid is introduced from an internal chamber in the screw, via holes in the screw. Thereafter, the channel once again decreases in the direction towards the outlet end. It is stated that the liquid that is delivered accordingly is expressed through the holes in the pervious plate of the inner wall, without creating any back pressure. The objective of the present invention is to provide a screw compressor and a method for compressing and washing matter, to achieve a more efficient washing of the matter that passes through the screw compressor and to more efficiently make use of such liquid that is introduced during the compression.